Earlier large research studies have shown that using hormone replacement therapy (HRT) increases breast cancer risk. A large study confirms the earlier results and offers more information on when risk increases, how different types of HRT affect risk, and how HRT affects the risk of different types of breast cancer.

Almost 68,000 postmenopausal women between the ages of 50 and 74 participated in this study, called the Cancer Prevention Study. The study started in 1992. The women's medical histories, including any HRT use and any breast cancer diagnoses, were kept track of for 13 years (until 2005). About half the women (47.6%) never used HRT. The other half either used HRT before enrolling in the study or used HRT at some point during the study. Some of these women used combination HRT (which contains estrogen and progesterone) and other women used estrogen-only HRT.

About 2,300 of the women were diagnosed with invasive breast cancer during the study. The researchers compared the number of women who used HRT and were diagnosed with breast cancer to the number of women who didn't use HRT and were diagnosed. The researchers also analyzed how different types of HRT and how long HRT was used affected risk. Combination HRT and estrogen-only HRT affected breast cancer risk differently.

Combination HRT:

Increased breast cancer risk, even when used for only a short time. Combination HRT increased the risk of the most common type of invasive breast cancer, invasive ductal carcinoma (IDC), by 75%. Combination HRT doubled the risk of invasive lobular carcinoma (ILC). Doubled risk means risk increased by 100%.

Breast cancer increased the most during the first 2 to 3 years of combination HRT use. Even though 2 to 3 years might seem like a short time to use combination HRT, it seems that most of the risk increase happened during the first few years of combination HRT use.

Breast cancer risk went down to average 2 years after women stopped using combination HRT.

Didn't increase the risk of invasive ductal carcinoma (IDC). Estrogen-only HRT did increase the risk of invasive lobular carcinoma (ILC) by 50%, but only when estrogen-only HRT was used for more than 10 years.

Menopausal side effects can dramatically reduce quality of life for some women. These women have to weigh the benefits of HRT against the risks. If you're having severe hot flashes or other menopausal side effects and are considering HRT, talk to your doctor about all of your options. Ask how you can minimize your breast cancer risk AND relieve your symptoms. Be sure to discuss the pros and cons of different types of HRT. The study reviewed here strongly suggests that estrogen-only HRT appears to increase breast cancer risk less than combination HRT. If you do decide to take HRT, ask if you can take a lower-dose formula and try to take it for the shortest time possible.